Several paths of evidence converge in implicating a role for the cholinergic system in the pathophysiology of affective illness. In both unipolar depressed and euthymic bipolar subjects, cholinomimetic drugs (i.e., muscarinic agonists, acetylcholinesterase inhibitors) exacerbate depressive signs and symptoms such as dysphoria, psychomotor retardation, impairment of attention and memory, hypothalamic pituitary adrenal axis hyperactivity and sleep EEG abnormalities (Janowsky et al., 1974; Overstreet, 1993). In healthy subjects, the acetylcholinesterase inhibitor physostigmine elicits a range of depressive symptoms including dysphoria, anergia, psychomotor slowing, emotional lability, sleep disturbances, memory and concentration impairment, and with higher doses, tearfulness and depression. These effects have been shown to reflect stimulation of muscarinic receptors (Davis et al., 1976; Oppenheim et al., 1979; Risch et al., 1981a). Cholinomimetics also exacerbate behavioral despair in putative animal models of depression. Conversely, the anticholinergic agent biperidine improved symptoms of depression in a placebo controlled study (Fleischhacker et al., 1987). Moreover, muscarinic cholinomimetics and a choline rich nutrient, lecithin (phosphatidylcholine) exert antimanic effects in bipolar subjects. Potentially consistent with these observations, depressed subjects exhibit hypersensitivity to cholinomimetic agents. Administration of muscarinic cholinergic agonists, ACh releasing agents or acetylcholinesterase inhibitors induce exaggerated effects on REM density and latency in depressed subjects than in healthy controls (Berger et al., 1983; Gillin et al., 1991a; Nofzinger et al., 1997; Nurnberger et al., 1989). In addition, both manic and depressed bipolar subjects show increased pupillary sensitivity to the muscarinic cholinergic agonist pilocarpine relative to controls (Sokolski and Demet, 1996). Despite the data implicating the mAChR receptor system in mood disorders, no direct in vivo investigations of the central mAChR have been performed in depressed subjects. A novel PET radioligand, [18F]FP-TZTP was recently developed by Eckelman (2001a; b) as a selective agonist of M2 receptors. Because the M2 receptor functions predominantly as a presynaptic release-controlling autoreceptor, decreased distribution volume (V) of this receptor could conceivably give rise to increased postsynaptic muscarinic receptor sensitivity. This project conducted a PET study of M2 receptor distribution volume in currently depressed subjects with major depressive disorder (n=18), currently depressed subjects with bipolar disorder (n=20), and psychiatrically healthy controls (n=22), about one-third of whom were studied in the past one year. The results confirmed the central hypothesis that M2 receptor V is decreased in regions where they are primarily located presynaptically in depressed subjects relative to healthy controls, namely in the cingulate cortex, insula, corpus callosum, and ventral striatum. These regions have been implicated in other studies as areas where impaired cholinergic regulation may result in abnormal emotional and attentional processing and altered emotional experiences including dysphoria, anxiety and euphoria. As such these data advance knowledge regarding the pathophysiology of depression. The magnitude of the cholinergic receptor abnormality correlated with the emotional salience which these subjects attributed to positively and negatively valenced words. Two scientific manuscripts have been prepared and submitted to describe these findings.